In the manufacture of semiconductor devices, photolithography is often used. Projection optics are used to image a mask or reticle onto a wafer. Optical systems having a refractive group have achieved satisfactory resolutions operating with illumination sources having wavelengths of 248 or 193 nanometers. As the element or feature size of semiconductor devices becomes smaller, the need for optical projection systems capable of providing enhanced resolution are needed. In order to decrease the feature size which the optical projection systems used in photolithography can resolve, shorter wavelengths of electromagnetic radiation must be used to project the image of a reticle or mask onto a photosensitive substrate, such as a semiconductor wafer.
Because very few refractive optical materials are able to transmit significant electromagnetic radiation below a wavelength of 193 nanometers, it is necessary to reduce to a minimum or eliminate refractive elements in optical projection systems operating at wavelengths below 193 nanometers. However, the desire to resolve ever smaller features makes necessary optical projection systems that operate at the extreme ultraviolet wavelengths, below 200 nm; and therefore, as optical lithography extends into shorter wavelengths (e.g., extreme ultraviolet (EUV)), the requirements of the projection system become more difficult to satisfy.
In a typical arrangement, a projection optics box (POB) contains the optical components that are used to reduce the image and form it on the photosensitive substrate (wafer). In most projection optical systems, mirrors that are carefully crafted to perform the intended functions are used in combination with a number of lenses arranged relative thereto. The mirrors serve to redirect the light in the projection optic box as it passes therethrough from the mask to the photosensitive substrate. Typically, the POB includes an arrangement of mirrors and lenses that are constructed and positioned to accomplish the intended result.
Conventionally, the mirrors interface with actuators, which serve to move and position the mirrors within the POB. One of the issues that has to be confronted when using such actuators is that the actuators typically have a coil and magnet construction with the coil being the active part of the actuator and the magnet being the passive part of the actuator that is directly connected to the mirror. One technique that can be used to filter out disturbances generated by the surroundings is the use of a reaction mass that is directly connected to the actuator. In order to keep the mirror in accurate position, the actuator constantly has to generate a counteracting force to all disturbance forces working on the mirror. However, the conventional reaction mass arrangements have associated disadvantages in that their constructions do not entirely filter out all types of disturbances and more specifically, some of the arrangements are unable to filter out disturbances (parasitic forces) generated in certain directions relative to the reaction mass.
What has heretofore not been available is an improved reaction mass arrangement that offers improved filtering characteristics for filtering out forces that are generated in a variety of different directions.